A variety of systems and apparatus are known for propelling boats. These systems include those disclosed in U.S. Pat. Nos. 763,684 to C. Manaker; U.S. Pat. No. 904,313 to G. Davis; U.S. Pat. No. 1,059,806 to A. Yarrow; U.S. Pat. No. 1,227,357 to H. Yarrow; U.S. Pat. No. 1,543,082 to B. Harley; U.S. Pat. No. 2,896,565 to G. Stevens; U.S. Pat. No. 3,440,743 to G. Divine; U.S. Pat. No. 3,745,963 to W. Fisher; U.S. Pat. No. 3,933,116 to F. Adams et al.; U.S. Pat. No. 3,980,035 to S. Johansson; U.S. Pat. No. 4,015,556 to A. Bordiga; U.S. Pat. No. 4,088,091 to R. Smith; U.S. Pat. No. 4,371,350 to C. Kruppa et al.; U.S. Pat. No. 4,406,635 to W. Wuhrer; U.S. Pat. No. 4,689,026 to M. Small; U.S. Pat. No. 4,713,028 to D. Duff; U.S. Pat. No. 4,977,845 to F. Rundquist; U.S. Pat. No. 5,046,975 to F. Buzzi; and U.S. Pat. No. 5,066,255 to R. Sand, the disclosures of which are hereby expressly incorporated herein by reference.
One particular class of such boat propulsion systems utilizes one or more surface-piercing propellers, typically mounted to a rear portion of the boat and extending downwardly into the body of water in which the boat is immersed. Surface-piercing propellers are often implemented in boat propulsion systems owing to their known ability to provide speed and fuel economy advantages on a planning boat hull. However, it is also known that such propellers do not operate optimally at all speeds, sea conditions, loading and trim, wherein propeller operation is generally affected by each and particularly affected by varying degrees of immersion, which refers to the amount of the propeller which is below the surface of the water.
It is therefore generally understood to be desirable with such boat propulsion systems to control the immersion depth of the one or more propellers such that the one or more propellers is immersed more deeply at low boat speeds, and is conversely immersed less deeply at higher boat speeds such as when the boat planes out. An example of one known propeller drive system 10 for controlling the depth of propeller immersion is illustrated in FIGS. 1 and 2. Propeller drive system 10 includes an articulating propeller drive assembly 12 extending from a rear 14 of a boat 16, and a surface-piercing propeller 18 mounted to an aft end of drive assembly 12. Drive assembly 12 includes a hinge 20, or ball assembly, wherein the immersion depth of propeller 18 may be varied by suitably actuating the hinge to thereby raise or lower the position of the propeller 18 relative to the boat 16 as indicated generally by arrows 22A and 22B. The angular limitations of the ball joint typically require a shaft extension of substantial length to produce an appropriate propeller height adjustment. Such propeller drive systems 10 are known to be used with a single propeller system, such as that illustrated in FIG. 1, or with a multiple propeller system, such as with twin propellers 18A and 18B as shown in FIG. 2. Propeller drive systems of the type illustrated in FIGS. 1 and 2, while generally effective in their intended purpose, are often complicated, expensive, unreliable and prone to mechanical failure. Moreover, such systems are typically difficult to operate and do not lend themselves well to automated control thereof.
Another known group of drive systems incorporates a tunnel in the bottom of the hull in which the propeller is partially or entirely enclosed within the tunnel, and in which some device adjusts the flow of water ahead of the propeller. To date, no such system proved successful in practical application. Surface-piercing propellers need to ventilate; that is, the portion of the propeller above the surface of the water needs to be exposed to atmospheric conditions or their functional equivalent. Existing systems generally lack adequate provision for the propeller to ventilate, or they incorporate complicated ducting arrangements forward of the propeller. Also, while the increased efficiency of a higher gear reduction ratio and associated larger propeller diameter is generally acknowledged, a propeller within a tunnel is size limited by both the hydrodynamic hull performance considerations which limit the cross-sectional area of the tunnel and by the need to maintain adequate propeller tip clearance, which typically may be on the order of 10% of the propeller's diameter.
What is therefore needed is a boat propulsion system that includes one or more operational advantages of the propeller drive system illustrated in FIGS. 1 and 2, but that does not suffer from the drawbacks associated therewith. What is desired, therefore, is a boat propulsion system in which a surface-piercing propeller of relatively unconstrained diameter, and preferably adaptable to disposition under the hull of the boat in plan view, is provided with adequate ventilation, is driven by a fixed, non-articulating shaft, and is variably immersed by means of simple, reliable, and relatively inexpensive components.